In general, an organic electroluminescent device (hereinafter referred to as organic EL device) includes a light-emitting layer and a pair of counter electrodes interposing the light-emitting layer therebetween in its simplest structure. That is, the organic EL device uses the phenomenon that, when an electric field is applied between both the electrodes, electrons are injected from a cathode and holes are injected from an anode, and each electron and each hole recombine in the light-emitting layer to emit light.
In recent years, progress has been made in developing an organic EL device using an organic thin film. In order to enhance luminous efficiency particularly, the optimization of the kind of electrodes has been attempted for the purpose of improving the efficiency of injection of carriers from the electrodes. As a result, there has been developed a device in which a hole-transporting layer formed of an aromatic diamine and a light-emitting layer formed of an 8-hydroxyquinoline aluminum complex (hereinafter referred to as Alq3) are formed between electrodes as thin films, resulting in a significant improvement in luminous efficiency, as compared to related-art devices in which a single crystal of anthracene or the like is used. Thus, the development of the above-mentioned organic EL device has been promoted in order to accomplish its practical application to a high-performance flat panel having features such as self-luminescence and rapid response.
Further, studies have been made on using phosphorescent light rather than fluorescent light as an attempt to raise the luminous efficiency of a device. Many kinds of devices including the above-mentioned device in which a hole-transporting layer formed of an aromatic diamine and a light-emitting layer formed of Alq3 are formed emit light by using fluorescent light emission. However, by using phosphorescent light emission, that is, by using light emission from a triplet excited state, luminous efficiency is expected to be improved by from about three times to four times, as compared to the case of using related-art devices in which fluorescent light (singlet) is used. In order to accomplish this purpose, studies have been made on adopting a coumarin derivative or a benzophenone derivative as a light-emitting layer, but extremely low luminance has only been provided. Further, studies have been made on using a europium complex as an attempt to use a triplet state, but highly efficient light emission has not been accomplished. In recent years, many studies centered on an organic metal complex such as an iridium complex have been made, as described in Patent Literature 1, for the purpose of attaining the high efficiency and long lifetime of light emission.
In order to obtain high luminous efficiency, host materials that are used with the dopant materials described above play an important role. A typical example of the host materials proposed is 4,4′-bis(9-carbazolyl)biphenyl (hereinafter referred to as CBP) as a carbazole compound disclosed in Patent Literature 2. When CBP is used as a host material for a green phosphorescent light-emitting material typified by a tris(2-phenylpyridine)iridium complex (hereinafter referred to as Ir(ppy)3), a relatively satisfactory light-emitting characteristic is exhibited. Meanwhile, when CBP is used as a host material for a blue phosphorescent light-emitting material, sufficient luminous efficiency is not obtained. This is because an energy level in the lowest excited triplet state of CBP is lower than that of a general blue phosphorescent light-emitting material, and hence triplet excitation energy of the blue phosphorescent light-emitting material transfers to CBP. That is, when the phosphorescent host material has higher triplet excitation energy than the phosphorescent light-emitting material, the triplet excitation energy of the phosphorescent light-emitting material is effectively confined, and as a result, high luminous efficiency is achieved. With the purpose of improving the energy confinement effect, in Non Patent Literature 1, the triplet excitation energy is increased by modification of structure of CBP, to thereby improve luminous efficiency of a bis[2-(4,6-difluorophenyl)pyridinato-N,C2′](picolinato)iridium complex (hereinafter referred to as FIrpic). In addition, in Non Patent Literature 2, 1,3-dicarbazolylbenzene (hereinafter referred to as mCP) is used as a host material to improve luminous efficiency on the basis of a similar effect. However, those materials are not satisfactory for practical use particularly from the viewpoint of durability.
In order to obtain high luminous efficiency, there are also needed balanced injecting/transporting characteristics for both charges (a hole and an electron). CBP is poorer in electron-transporting ability than in hole-transporting ability, and hence the balance between charges in the light-emitting layer is disturbed. As a result, excessive holes flow out to the cathode side to reduce a recombination probability in the light-emitting layer, resulting in a reduction in luminous efficiency. Further, in this case, a recombination region in the light-emitting layer is limited to a small region in the vicinity of an interface on the cathode side. Accordingly, when an electron-transporting material, such as Alq3, having a lower energy level in the lowest excited triplet state than Ir(ppy)3 is used, there may occur a reduction in luminous efficiency due to transfer of the triplet excitation energy from the dopant to the electron-transporting material.
As described above, in order to obtain high luminous efficiency in an organic EL device, there is needed a host material that has high triplet excitation energy and balanced injecting/transporting characteristics for both charges (a hole and an electron). Further desired is a compound that is electrochemically stable, has high heat resistance, and has excellent amorphous stability, and hence a further improvement has been demanded.
In addition, as an organic EL device using a cyclic compound containing Si in its ring, in Patent Literature 3, there is disclosed an organic EL device using the following compound (a1) as a hole-transporting layer material.

In addition, as an organic EL device using a cyclic compound containing Si in its ring, in Patent Literature 4, there is disclosed an organic EL device using the following compound (a2) or (a3) as a hole-transporting material. In the formulae, X10, X11, X16, and X17 each represent CR2, O, S, NR, SiR2, or GeR2, and R8 represents a substituted or unsubstituted alkylene group or a divalent aromatic hydrocarbon group. However, the compound (a2) means a trimer, and the compound (a3) means a dimer linked through a hydrocarbon group.
